1. Field of the Invention
This invention relates to semiconductor devices, and more particularly to silicon carbide junction field effect transistors (JFETs) having low gate leakage for high temperature applications.
2. Description of the Related Art
Silicon carbide (SiC) is a crystalline substance that can withstand very high temperatures. Semiconductor devices manufactured of SiC can withstand temperatures in excess of 200.degree. C. Thus, SiC semiconductors are desirable for applications that require exposure to high temperatures.
SiC electronics technology can be viewed as a means of controlling flame temperature by reducing fuel-to-air ratios in high temperature environments, such as encountered in the combustion of a stationary gas turbine or gas turbine employed in an aircraft jet engine, which reduces the production of nitrogen oxide (NO.sub.x) emissions. When the operating temperature is excessively high, NO.sub.x emissions, which are classified as pollutants, are also excessively high. However, when fuel is burned lean to hold down the flame temperature, the flame can produce too much CO and become unstable or even be extinguished. The design constraints for these turbine combustors have become so exacting that the manufacturing tolerances are difficult to achieve. An apparatus for addressing these problems by using a closed loop control of the flame temperature is disclosed in commonly assigned D. M. Brown et al., "Combustion Control for Producing Low NO.sub.x Emissions Through Use of Flame Spectroscopy," U.S. application Ser. No. 07/878,933, filed May 5, 1992, which is herein incorporated by reference.
High temperature sensors can be used for monitoring and controlling the chemical content of the turbine exhaust gases, but electronic amplifiers for amplifying the sensor signals must be situated at some distance from the combustion zone and exhaust to avoid performance degradation or destruction on account of the high temperatures near the combustion zone. Detrimentally, the signal level from the sensor is degraded by noise components in the connective wiring as the signal propagates from the sensor to a remote amplifier. As a result, the signal-to-noise ratio (SNR) is reduced. Preferably, a sensor and an amplifier are combined so that little degradation of the signal occurs as the signal passes from the sensor to the amplifier. Advantageously, a properly designed SiC semiconductor amplifier is capable of withstanding the high temperatures near a turbine exhaust outlet. Therefore, use of a combined SiC based semiconductor amplifier and sensor will result in improved turbine flame monitoring and control apparatus.
A SiC sensor is disclosed in commonly assigned D. M. Brown et al., "Silicon Carbide Photodiode with Improved Short Wavelength Response and Very Low Leakage Current," U.S. application Ser. No. 07/878,937, filed May 5, 1992, which is herein incorporated by reference. A SiC metal-oxide-semiconductor field effect transistor (MOSFET) is disclosed in commonly assigned D. M. Brown et al., "Silicon Carbide MOSFET Having Self-Aligned Gate Structure and Method of Fabrication," application Ser. No. 07/925,823, filed Aug. 7, 1992, which is herein incorporated by reference. MOSFETs generally exhibit a high gain-bandwidth product and high input impedance, which are desirable for amplifying low level voltages. However, operating voltages required for reliable MOSFET device operation at very high temperatures must be kept very low because of the time dependent breakdown of the insulating layer, and therefore the voltages for reliable operation in the desired temperature range are not high enough for all of the applications desired.